A self-powered electrolytic process for glucose to hydrogen conversion (2024)

The integrated device

This integrated device is schematically shown in Fig.1 and the photographs of experiment setup are shown in Supplementary Fig.1. The device includes four key units: REACTOR, liquid-catalyst fuel cells (LCFCs), polymer exchange membrane electrolytic cell (PEMEC), and SHAREDCELL.

Conception of the integrated device. a Schematic illustration of the integration of REACTORS (stored glucose-POM solution and H₃PO₄ solution in the left and right side respectively), LCFCs (biomass fuel cell stack), PEMEC (hydrogen electrolyzer) and SHALLEDCELL (the tank sored Fe²⁺/Fe³⁺ solution in the middle), and b the detail electron flow analysis in the device

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The feedstock of glucose is mixed with POM catalyst in the REACTOR. A simple Keggin-structure POM catalyst, phosphom*olybdic acid solution (noted as PMo₁₂) was used in this study. In the REACTOR unit, glucose is oxidized and degraded to CO₂ by oxidization of PMo₁₂ under thermal heating

As a result, one \([PMo_{12}]^{3 - }\), noted as [POM]³⁻, captures one electron and is reduced to \([PMo_{12}]^{4 - }\) (noted as [POM]⁴⁻), and simultaneously one proton ion is released from glucose. It should be noted that the above reaction is based on the assumption that one PMo₁₂ will receive only one electron, (i.g. the reduction degree is 1). Actually, one POM can receive more than one electron because their 12 Mo^VI ions in one POM molecules. The electrons that one mole POM received during the reaction is called reduction degree^23,24. In our experimental condition, the reduction degree of POM is in the range of 2–3.5 depending on the reaction time and temperature, concentration etc.

The LCFC is similar to the devices reported in our previous studies^23,24,25, which was constructed using a Nafion 115 membrane sandwiched between two 3D graphite electrodes with no metal loading. The reduced [POM]⁴⁻ from the REACTOR functions as anode electrolyte which supplies electrons to the carbon anode in the LCFC, and Fe³⁺ functions as cathode oxidation agent, as shown in Fig.1. It is noted that the [POM]³⁻/[POM]⁴⁻ redox pair at a concentration of ~0.5 M has an electrochemical potential ranges from +0.38 to +0.45 V relative to the standard hydrogen electrode (SHE)²⁶, and Fe³⁺/Fe²⁺ redox pair has a standard electrochemical potential +0.77 V. Clearly, Fe³⁺ can oxidize reduced [POM]⁴⁻ to [POM]³⁻, and itself will be reduced to Fe²⁺. Therefore, a fuel cell consist of two redox pairs of [POM]³⁻/[POM]⁴⁻ and Fe³⁺/Fe²⁺ can be fabricated, as shown in Fig.1. Practically, [POM]⁴⁻ at anode side gives the electrons through external circuit to cathode side, and simultaneously releases protons as charge-balancing ions. The protons are then penetrates through Nafion membrane to the cathode cell where Fe³⁺ captures the electrons to form Fe²⁺. Meanwhile, the feedstock of glucose in the REACTOR continuously reacts with the regenerated [POM]³⁻ to keep a stable reaction state. (The detail electro-chemical reactions in the entire device will be discussed later).

The PEMEC is sandwiched between a simple carbon anode without coating any catalyst and a carbon cathode coated with Pt black catalyst (ca. 2 mg cm⁻²) for hydrogen evolution. There is a SHAREDCELL between LCFC and PEMEC in which Fe³⁺/Fe²⁺ pair is used as cathode electrolyte for LCFC and anode electrolyte for PEMEC. As discussed above, Fe³⁺ ions are oxidization agent in the LCFC (on cathode), but Fe²⁺ ions in the SHAREDCELL functions as a reducing agent in PEMEC (on anode) which release electrons to external circuit and are oxidized back to Fe³⁺ ions. The electrons and protons from anode of the LCFC are transferred to the cathode of PEMEC in which they combine to form hydrogen gas.

The SHAREDCELL refers to the SHAREDCELL between LCFCs and PEMEC, in which the Fe³⁺/Fe²⁺ electrolyte solution is shared by the cathode of LCFCs and the anode of PEMEC. As discussed above, the open cell voltage of one fuel cell is ~ 0.35 V, which is not high enough to split water in an acid PEMEC solution. Therefore, multi-fuel cells are assembled together in series to form a cell stack.

Using a three fuel cell stack as an example, the actual electronic circle is shown in Fig.1b. When one electron is given from the LCFCs stack, there would generate one Fe²⁺ ion at cathode side and one H⁺ ion at anode side in each single LCFC. When this electron is transferred to PEMEC unit, one H⁺ ion is consumed to ½H₂ in PEMEC’s cathode. Simultaneously, one Fe²⁺ ion is oxidized to Fe³⁺ in PEMEC’s anode. According to Fig.1b, it is clear that to reduce one H⁺ to 1/2H₂, one electron is transferred, but three H⁺ and three Fe²⁺ ions are produced for a LCFC stack with three individual fuel cells, which means two extra H⁺ ions and Fe²⁺ ions will be produced for producing 1/2H₂. In order to balance the charge and ions for a stable process, O₂ or air is pumped to the SHAREDCELL to react with excess Fe²⁺ and H⁺ ions as the follows:

As shown in Fig.1, by combing four key units of REACTOR, LCFCs, PEMEC and SHAREDCELL together, both polyoxymetalate and iron ions can be completely regenerated, so the concentrations of Fe³⁺/Fe²⁺ and [POM]³⁻/[POM]⁴⁻ are kept in steady state during the process in the integrated device. For an ideal case in which the glucose is completely consumed and the reactions involved in the entire system (three fuel cell stack) are

LCFC: For each individual fuel cell (fuel cell-1, 2, and 3):

Therefore, for a stack of three fuel cells in a series, the reaction will be

LCFC STACK:

PEMEC:

SHAREDCELL:

THE NET REACTION: (the sum of Eqs. (3), (6)–(10))

It should be noted that at ideal condition, the H₃PO₄ in the cathode cell only serves as buffer which will not be consumed, and all H⁺ are actually from the oxidation reactions of biomass and water in the anodes of the fuel cell stack, as shown in Eq. (3).

It should be noted that our integrated device is different from any previous reported. It is not a simple physical connection of an independent fuel cell with an independent electrolyzer using electric wires. Instead, the fuel cell and electrolyzer are dependant one to another by using a SHEREDCELL. If it is a simple physical connection of an independent fuel cell and an electrolyzer, the electrolytes in both cells have to be regenerated using external electric power or chemical reaction. However, by using a SHAREDCELL, the consumed Fe²⁺ can be self-regenerated as indicated by the Eqs. (3)–(10). As a result, no external electricity is needed in a continuous operation process. This type of integrated process and device has not be reported previously.

To speedup the reactions, the entire solution was heat to 85 °C. It should be noted that because no further cold water will be added to the cells but only solid sugar is continuously fed to the cells, the energy used for heat the solution to 85 °C is only a one time requirement. If the reaction tank is well insulated, no further heat is needed to maintain the temperature at 85 °C. In other words, there will be no thermal energy consumption during the process if a thermal insulation is used.

Performance and stability of the integrated device

To determine how many fuel cells in a series can provide enough input voltage and current to PEMEC for hydrogen evolution, the fuel cell number and their performance in a LCFC stack was firstly studied. From Fig.2a, the measured open-circuit voltage in a single cell of LCFC was ca. 0.33 V. When different number of single cells in series was assembled into a LCFC stack, the open-circuit voltage increased linearly (black bars in Fig.2a). However, when the LCFCs unit was connected with a PEMEC unit to form an integrated device, the LCFCs output voltage (=PEMEC input voltage, red bars in Fig.2a) was lower than the open circuit voltage.

Design of LCFCs stack. a Effect of single cell numbers of LCFCs on open-circuit voltage in separate LCFCs unit and LCFCs output voltage (=PEMEC input voltage) in the integrated device at 20 °C, respectively; the blue broken line marks the onset applied voltage value of 0.72 V in PEMEC. b Polarization curve in separate PEMEC unit at 20 °C; the black line is the experimental I–V curve in a PEMEC, and the red broken line is to determine the onset applied voltage in PEMEC. c Voltage–current plots in separate LCFCs unit with different total electrode area at 20 and 85 °C, respectively; d Voltage–current plots in separate LCFCs unit with electrode area of 12 cm² at different running temperatures

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For the purpose of understanding the PEMEC performance, a separated test of PEMEC unit under different input electric fields (power was provided by an electrochemical workstation rather than LCFCs) was conducted and the I–V curve is shown in Fig.2b. It can be found that the onset applied voltage in PEMEC was 0.72 V, suggesting that hydrogen could start to form at a supplied voltage greater than 0.72 V. It should be noted that although the standard water split voltage is 1.23 V, the actual reaction of this PEMEC is not pure water split but the combination of reaction (6) and (7) so the electrolysis voltage of this PEMEC is much lower than pure water split. Comparing with the results of Fig.2a, we concluded that at least four single LCFCs should be connected in series to maintain an output voltage higher than 0.72 V (the critical voltage for electrolysis of PEMEC).

The output currents of LCFCs stack are affected by the total electrode area and running temperature. As shown in Fig.2c, while increasing the total electrode area of LCFCs from 4 to 12 cm², the output currents of cell stack increased a lot at running temperature of both 20 and 85 °C. But further increasing total electrode area to 36 cm², the output current was improved little. From Fig.2d, the output current of LCFCs with total electrode area of 12 cm² increased significantly while the cell running temperatures were increased. The similar results were observed for LCFCs with 4 and 36 cm² electrode areas, as shown in Supplementary Fig.2. Because the increase of cell running temperature results in an increase of internal energy of electrolytes and a decline of activation polarization²⁷, higher running temperature in LCFCs leads to lower ionic resistance and higher redox rate, so an increase in the output currents of cell stack.

A PEMEC with 1 cm³ cell volume and a LCFCs stack with 4 single cells and 12 cm² total electrode area were combined together and constructed into an integrated device, which was used to perform the continuous test running at 85 °C for 4 h, as shown in Fig.3. In Fig.3a, measured LCFCs output voltages slightly fluctuated around the value of 0.83 V during the test, indicating the output voltage of LCFCs in the integrated device could keep stable. In other words, the PEMEC unit can be afforded stable input voltage from LCFCs to maintain hydrogen evolution within 4 hour testing time. To fully test the device performance and the stability, long time tests should be done in future.

Stability test of the integrated device. a measured LCFCs output voltage (equals to PEMEC input voltage) vs. time; the black points are experimental output voltages of LCFCs, and the red broken line marks the output voltages fluctuating around the value of 0.83 V. b molar ratio of Fe³⁺/Fe²⁺ vs. time; the red broken line marks the Fe³/Fe²⁺ ratios keeping stable around a value of 9. c measured current in the closed electric circuit vs. time; the red broken lines mark the currents fluctuating within a narrow range of 7.3–8.2 V. And d measured H₂ yield vs. time

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The absorbance of Fe-phenanthroline complex at the wavelength of 510 nm can be utilized to determine Fe²⁺ concentration because of the linear relationship between the absorbances at 510 nm and Fe²⁺ concentrations, as shown in Supplementary Fig.3a. The measured Fe²⁺ concentrations in the Fe³⁺-Fe²⁺ solution tank during test are shown in Supplementary Fig.3b. From measured Fe²⁺ concentrations, the corresponding molar ratios of Fe³⁺/Fe²⁺ could be calculated, which are shown in Fig.3b. The measured Fe³⁺/Fe²⁺ ratios dropped quickly at the beginning of the test, then kept stable around a value of 9. It should be noted that the ratio of Fe³⁺/Fe²⁺ in the SHAREDCELL unit is critically important. According to Nernst Equation,

if the ratio of Fe³⁺/Fe²⁺ is too high, the electrochemical potential of Fe³⁺–Fe²⁺ solution in SHAREDCELL unit would be also high. As a result, higher voltage for electrolysis in PEMEC is required for hydrogen formation. In other words, the reaction rate of Fe²⁺+H⁺=Fe³⁺+1/2H₂ in PEMEC unit will be slowed down, and eventually, Fe²⁺ will not be regenerated to Fe³⁺, and the hydrogen production will be stopped. On the other hand, if the ratio of Fe³⁺/Fe²⁺ is too low, the Fe³⁺-Fe²⁺ solution potential in SHAREDCELL unit would be low too. Thus, because the output voltage of LCFCs equals to the difference between electrochemical potential of Fe³⁺/Fe²⁺ and [POM]³⁻/[POM]⁴⁻ pairs, the LCFCs stack will not provide high enough output voltage (>0.72 V in this study) for hydrogen production. Therefore, the steady ratio of Fe³⁺/Fe²⁺ redox pair in SHAREDCELL unit is very important to maintain the stable hydrogen yield by the whole integrated device system. The recycling of Fe³⁺ is partially contributed by the self-regeneration in anode of electrolytic cell and at the same time by the oxidation reaction with oxygen in air. Although the oxidation rate of Fe²⁺ by O₂ is relatively slow in acid solutions, it can been speeded up by using co-catalyst such as Cu–SO₂ which has been reported in previous research²⁸_. Our calculation indicates that with the co-catalyst, the regeneration rate of Fe³⁺ could be faster than Fe²⁺ consuming rate in the SHAREDCELL (see Fig.1). In this study, because the total reaction time is a few hours and no obvious Fe²⁺ concentration change was noticed, so the co-catalyst Cu–SO₂ was not used.

As shown in Fig.3c, the current (equals to LCFCs output current or PEMEC input current) of the integrated device could almost keep stable during the test with a small fluctuation within a narrow range of 7.3–8.2 mA. This result not only confirms the stable regeneration circle of Fe³⁺/Fe²⁺ in SHAREDCELL unit, but also proves the stable equilibrium between releasing electrons at LCFCs anode and capturing electrons at PEMEC cathode.

The producing H₂ gas was collected and measured by water displacement method. In Fig.3d, it can be found that the H₂ production rate remained stable during the test, indicating the transferring proton rate could keep a steady dynamic equilibrium with the receiving electron rate to continuously form H₂ gas at PEMEC cathode. Moreover, it can be further inferred that the reaction rate of glucose to release protons and electrons in REACTOR is fast enough to provide a stable hydrogen production rate at PEMEC cathode during the stability test.

Furthermore, as shown in Fig.3d, a steady pure hydrogen production rate of 0.0432 mL min⁻¹ based on 1 cm³ cell volume of PEMEC (about 62.2 m³ H₂/m³/d based on PEMEC volume) can be obtained in our integrated device, which is almost 26 times higher than that of MEC-based system using glucose reported in literature²². The higher hydrogen production rate in our integrated device could be attributed to using POM catalyst to substitute for exoelectrogenic microbes, because POM catalysts have higher reaction activity with glucose than microbe catalysts²⁴.

The total electrons transferred from LCFCs to PEMEC via external circuit can be obtained from the current-time integral area in Fig.3c. After 234 minutes running time, 108.2 columbus electrons were transferred in the integrated device. In case of the ideal condition that all the transferred electrons are captured by protons to produce hydrogen gas, the theoretical yield of H₂ could be calculated as 0.561 mmol (=12.56 mL) during the test according to Faraday-Matteucci’s laws:

Experimentally, the measured H₂ yield was 9.96 mL during the test, as shown in Fig.3d. Therefore, the Faraday efficiency,defined as theratio of measured to theoretical yield of H₂,was 79.3%, which means that 79.3% of the electric current generatedfrom the feedstock glucose is used to produce hydrogen by PEMEC in the integrated device.

Glucose functions as fuels that provides energy to drive LCFC and hydrogen donor for hydrogen production in PEMEC. However, glucose was not directly oxidized on graphite anode electrode because of lack of catalyst for glucose oxidation. POM reacted with glucose and worked as charge carrier that transfers electrons from glucose to electrode (verified by cyclic voltammogram (CV) of POM-glucose solution, as shown in Supplementary Fig.5). In order to investigate final products of glucose decomposition with PMo₁₂ after continuous running, the glucose-PMo₁₂ solution was continuously heated under reflux in 85 °C water bath for over 10 h under N₂ atmosphere. The liquid samples before and after reaction were characterized by ¹H-NMR. It is known that the native D-glucose has only two anomers, generally cited as 36% for the α-D-glucose and 64% for the β-D-glucose²⁹. From the ¹H-NMR spectra (shown in Fig.4 a, b), it can be observed that after the long-time reaction with PMo₁₂ at 85 °C, the specific peaks assigned to α- and β-D-glucose completely disappeared and the peaks assigned to alcoholic hydroxyl group at 3.1 ∼ 3.9 ppm were also almost disappeared, but only a new peak arose at 7.8 ppm assigned to aldehyde group^24,29. The result indicates the two glucose anomers firstly changed into open-chain structure and then were oxidized to low molecular derivatives with aldehyde groups, as shown in Supplementary Fig.4. Previous researches also confirmed that the major products of biomass (e.g. starch, glucose and cellulose) reacted with POM catalysts in aqueous solution were aldehydes and organic acids^23,24,26. In addition, the emission gas from the reaction was collected using a sampling gas bag and analyzed by gas chromatography (GC). As shown in Fig.4c, carbon dioxide was the only emission gas product and its percentage was 1.3% that is significantly higher than the value in dry air (0.04%), indicating that glucose was oxidized to CO₂ by PMo₁₂. Because glucose was used as the only feeding raw material, the total organic carbon (TOC) analysis showed that 88% weight of the initial 5.4 g of glucose was degradated to CO₂ after 10 h reaction with 100 mL of 0.3 mol L⁻¹ of PMo₁₂ at 85 °C (shown in Fig.4d). During the glucose oxidation porcess, POM maintains the integrated structure (verified by UV–vis spectra shown in Supplementary Fig.6) because it is a roubust and self-healing catalyst, hundreds of thousands of turnovers are possible^30,31,32.

Glucose degredation with PMo₁₂ for long time reaction. a, b ¹H NMR spectra of liquid samples before and after reaction (solvent D₂O); c composition analysis of emission gas by GC; d TOC percentage of liquid samples before and afer reaction

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The adopted PMo₁₂ catalyst is tolerant to catalyst-poisoning contaminations because POMs are robust and self-healing^33,34,35. Borras–Almenar et al. also indicated that for the reaction mixture containing the substrate and the POM catalyst, hundreds of thousands of turnovers are possilbe³². So the PMo₁₂ catalyst can be continousely regenerated and used in this integrated device. As a result, if the reaction is continuous long enough, it is believed that glucose will be completely degredated to CO₂ eventually.